Containers holding liquids and bulk solids are economically manufactured in a continuous blow molding process wherein a parison comprising a hollow tube of molten polymer resin is extruded continuously from a flow head. The parison is acted on by a series of moving molds, each of which is formed of mold halves which sequentially engage a respective portion of the parison by closing about the parison from opposite sides. As the mold halves comprising a particular mold close about a parison portion, knives on the mold halves sever the parison portion from the continuously extruding parison. The mold then moves away from the flow head to allow the next mold to engage its respective parison portion. After a parison portion is engaged by a mold and cut from the parison, air is injected into the parison portion forcing it to expand and assume the shape of the mold. The mold is then opened to release the newly molded container to a conveyor, which transports the container for further processing. The mold then travels back to the flow head to mold the next container. A detailed description of a process and apparatus for producing the containers is provided in U.S. Pat. No. 5,840,349, hereby incorporated by reference herein.
It is advantageous to provide containers formed of multiple layers of different plastic resins having different characteristics. For example, a container holding food stuffs may have an innermost layer which is inert and will not react chemically with the container contents, an intermediate layer which is impermeable to oxygen to prevent the contents from oxidizing and an outermost layer which has a particular color, or is resistant to abrasion.
To produce such a multi-layer container, the parison from which the container is molded is extruded with multiple layers. This is accomplished in the flow head, which has multiple ports for receiving different polymer resins. The flow head has a central bore within which a mandrel is positioned, thereby forming an annular space. The different polymer resins are forced under pressure through the ports and into the annular space at different positions along the length of the flow head. In the three layer example described above, the resin that will form the innermost layer enters the annular space farthest upstream and conforms to the annular space to form a tube. This resin tube continues to flow through the flow head, and the oxygen impermeable resin enters the annular space through a port positioned further downstream. The oxygen impermeable layer is forced into the annular space and forms a second tube surrounding the aforementioned tube (the annular space is larger to accommodate the second layer). Finally, the colored resin that will form the outermost layer is introduced to the annular space through a third port downstream from the second port, and forms a third tube surrounding the first two tubes. The three layer parison thus formed exits the flow head continuously, and portions of the parison are captured by the molds in a continuous process as described above to produce the containers.
It is often desired to apply a layer which varies in thickness as a function of length along the container, or in the limiting case, a layer which extends over only a portion of the container. For example, to effect a color transition on the outermost layer so that the bottom portion of a container is one color and the top portion is another color, resin having the desired color of the container bottom portion may be injected into the annular space of the flow head to initially form a relatively thick outer resin layer which becomes gradually thinner as the parison traverses the flow head. The color of the parison will gradually change over its length, initially displaying the color of the relatively thick outer layer, and changing color gradually as the outer layer becomes thinner and the color of the underlying layer becomes visible through the thinning outer layer. The injection of a resin layer which varies in thickness produces a parison having varying characteristics, and if containers having consistent characteristics are to be produced, the injection of the varying thickness layer must be synchronized with the molds such that each mold intercepts the parison at the same position relative to the varying thickness layer. In the example provided above, this synchronization will ensure that the bottom portion of all the containers is one color, the top portion is a second color, and the transition between the top and bottom portions occurs at the same location on the container. In the extreme case, the varying layer may be an intermittent layer wherein the thickness of the varying layer drops to zero abruptly.
Prior art methods and devices for producing containers having resin layers of varying thickness have low production rates. This is due in large part to the synchronization requirements, coupled with the flow characteristics of the resin. Production rates under 40 container per minute are achieved by some prior art devices. It would be advantageous to be able to produce containers as described above at higher production rates.